Do I need a new disc-brake proportioning/centering valve?

[cris well]

I've been having problems with my C-10 68 converted to 71 disc setup.

The truck will stop but the brake pedal is really hard to push and It won't lock the brakes down if needed--just slows the truck down.

I've searched and read through many threads and think I may have found the problem.

My proportioning valve button is pushed in and stuck. First I stomped on the pedal with the front lines un-hooked and all the fluid out of the master-cyl. with the key switch on (didn't work).

Then I took off the prop. valve and tapped on it really hard from all angles (didn't work).

Then I blew through the front brake line with an airline while holding my finger over the front outlet on the bottom of it and the button will pop out while the air is on it but pops right back in when I take the air off.

Should I put it back on and try it or should I get a new one?



I've already put a new master-cyl.and booster on it, and was in the process of putting new brake hoses on it when I started looking in to the prop. valve. I'm gonna put the new hoses on and try it again I think.

[jocko]

is the valve from the 68 or the 71?

reset of the brake differential pressure valve can be accomplished by several very firm pushes on the brake pedal, if it's not broken.

did you use the 68's manual brake lines or plumb new lines? Disk brakes systems sometime use smaller dia brake lines.

You mentioned a new MC - did you install a disk/drum master (ala the 71 donor) or buy a new 68 dual res (which is only for a drum/drum setup)?

Was your 68 power or non-power drum/drum? If non-power is your push rod adjusted appropriately to be just barely not engaging the booster when the pedal is against it's rest stop? (this probably isn't related to your problem though)

Maybe post a little more detail of what you swapped over from the 71, if you swapped everything over, that'd be better, mix/match 68 and 71 stuff might be part of the porblem since there is more to the whole system than just the mc and booster. With a little more info, might be able to narrow it down.

[RichardJ]

The unit you are talking about is a combination valve. It has three separate valves combined into one unit.
The section with the pin is called a "Metering Valve", but sometimes it is called a "hold-off valve". Its job is to hold-off pressure going to the front brakes untill the MC pressure exceeds a certain level. This allows pressure to the rear brakes so they can begin to slow the truck, before the front brakes begin their job. This is an anti-dive feature.

The fact that the pin comes out with air pressure means that the metering valve is working. The pressure to start opening the metering valve is around 40 psi, but is higher on some vehicles.

If the front brake lines are full of air, the air will compress and the pressure will never be high enough to open the metering valve far enough to bleed properly.

There is a special tool that will hold the pin out far enough, to get enough air out to get the bleeding started. Once most of the air is out, there will be enough pressure to hold the metering valve open for proper bleeding.

I use a vacuum pump for bleeding and it will eventually pull enough fluid through the metering valve.

There are two types of metering valves used in different combination valves. The other type requires the pin to be pushed IN.
As good or better reference as any.http://www.autozone.com/autozone/rep...00c1528004b46f

Keep bleeding, you'll get it.

[cris well]

The disc-brake swap was already done when I got the truck.

I've learned by reading through some other threads that on a drum brake set-up the front lines run behind the motor cross-member and on a disc set-up they run behind it--Mine is behind it.

Some of the stuff is mismatched from 68 to 71--the center drag link is from the 68 I found when putting new tie-rod ends on it.

The valve is from a disc-brake truck as well as the master cyl. and the booster. I bought them for a 71 when I bought them. (I've also put new calipers on it, I forgot to mention that).

As far as if it was originally a power drum truck I don't know--Is there any way to tell? I hadn't thought about the push rod being adjusted wrong.

So....I am about to put my prop. valve back on it and bleed them out and see what it's like now.

[cris well]

Here are pics of the prop valve and the firewall where the booster bolts up--maybe someone can tell if it had a booster originally for the 4 wheel drums.

[treveiger]

Interested.
Posted via Mobile Device

[cris well]

I tried bleeding yesterday with my old prop valve back on it with the button still stuck in.

I started at the right rear with an Advance Auto rental vaccum-tool and got nothing. Then I put a clear hose in a bottle of fluid from the bleeder and pumped it--no bubbles or fluid coming through the tube.

Then I left the bleeder out all night on the right rear still nothing. I'm thinking the rear rubber hose may be stopped up.

[jocko]

Last statement concerns me - you left the bleeder unscrewed overnight, do I understand that correctly? Technically speaking, the valve should only be opened when the system is pressurized (i.e. pedal pressed by someone, suction/vacuum placed at the wheel cylinder end, or pressure placed at the master cylinder end (like a sealed Powerbleeder unit, etc)). Leaving the valve open only introduces air into the system from the far end, not a good idea. Same applies when pumping the brakes to bleed, the valve should be opened just as the pedal is pushed, and closed immediately after the full stroke of the pedal (before it is allowed to raise) - i.e. just a pressurization, no vacuum (if you leave the valve open when the brake pedal is released, it just sucks air back into the system again and you kinda get nowhere). It takes patience on that first far wheel, if the system has a lot of air or was run dry/lines changed, etc - takes a bit to get fluid back there.

If the valve is stuck closed on one side - presumable block rear drum flow - did you attempt to bleed the fronts, even out of the normal order? If fronts bleed easily and rears get nothing, then that's another indication you need a new valve.

Once you reinstalled the valve, how does your pedal feel? If you were not able to get the button to un-stick by some heavy brake pumps (with all bleeders closed) then you probably need a new valve. From the pics, that looks like a factory setup. I do not believe combo valves were installed by factory, just available from aftermarket - but otheres may have more detail on that than I do.

[cris well]

Hi jocko,

Yes I took the bleeder out to gravity feed it last night and never got anything out of it. When gravity feeding should i put a block on the pedal to hold it down?

I also thought about doing the fronts first since the button on the prop valve is stuck in. I was thinking that since you normally have to hold the button in with a tool to build up pressure in the back maybe I should do the front first??

And yes it is a factory set-up off of a 71 (I'm pretty sure)--It was already done when I got the truck.

[406 Q-ship]

Originally Posted by jocko

Last statement concerns me - you left the bleeder unscrewed overnight, do I understand that correctly? Technically speaking, the valve should only be opened when the system is pressurized (i.e. pedal pressed by someone, suction/vacuum placed at the wheel cylinder end, or pressure placed at the master cylinder end (like a sealed Powerbleeder unit, etc)). Leaving the valve open only introduces air into the system from the far end, not a good idea. Same applies when pumping the brakes to bleed, the valve should be opened just as the pedal is pushed, and closed immediately after the full stroke of the pedal (before it is allowed to raise) - i.e. just a pressurization, no vacuum (if you leave the valve open when the brake pedal is released, it just sucks air back into the system again and you kinda get nowhere). It takes patience on that first far wheel, if the system has a lot of air or was run dry/lines changed, etc - takes a bit to get fluid back there.

If the valve is stuck closed on one side - presumable block rear drum flow - did you attempt to bleed the fronts, even out of the normal order? If fronts bleed easily and rears get nothing, then that's another indication you need a new valve.

Once you reinstalled the valve, how does your pedal feel? If you were not able to get the button to un-stick by some heavy brake pumps (with all bleeders closed) then you probably need a new valve. From the pics, that looks like a factory setup. I do not believe combo valves were installed by factory, just available from aftermarket - but otheres may have more detail on that than I do.

You can barly crack open the bleeders with the cover off the MC to do a gravity bleed. It works when you want to do a faster pressure bleed on a completely dry system. As long as the brake pedal is not depressed and allowed to return with the bleeders open, it is the return of the pedal that sucks the air into the system. There is one thing to keep in mind when doing a gravity bleed is that it not a complete bleed and a normal pressure bleed is still required.

cris-well being you don't know what parts are mismatched, put a new MC and combination valve for a 1971 Power Disc Brake truck. Then your sure.

[jeremy32v]

i have the same problem with my 70, with 71 disc front end. great info!

[slikside]

subscribed....will be tackling this issue soon.

[cris well]

I don't know if it will help jeremy32v or slikside but you may wanna start with the rear rubber brake hose.

After going through everything else I finally decided after reading someone else's post about a stopped-up rear hose to check mine and damn if that wasn't my problem.

I had already checked or changed everything else and my new proportioning valve is still stuck with the button in but I have much better brakes now and they will lock up to a full skid when I want.

I'm gonna try to bleed them again starting with the front brakes then the rear to see if I can get the new valve to center. The brake light will go out for a little while and then it will come back on--I think it still has some air in the lines.

Thanks to everyone for all the help and suggestions you have given me with mine.